Screw and rod fixation assembly and device

ABSTRACT

The present invention provides for a screw and rod fixation assembly for fixing a screw and, optionally, a rod. The screw and rod fixation assembly includes a screw, fixing mechanism, a substantially annular ring, rod seating mechanism, and locking mechanism. The present invention also provides for a fixing mechanism for fixing a screw, wherein the fixing mechanism further includes an inner surface wall having a gripping portion and a non-gripping portion. Further, the present invention provides for a substantially annular ring for guiding and providing mechanical and frictional force to a screw head. Additionally, the present invention provides for a rod seating mechanism operatively engaged to the screw head and including at least one flexible portion capable of being compressed against a portion of a rod therein. Finally, the present invention provides for a locking mechanism for engaging the rod and the rod seating mechanism. The locking mechanism includes a deflecting mechanism for deflecting the at least one flexible portion of the rod seating mechanism against and around the rod as the locking mechanism further engages the at least one flexible portion of the rod seating mechanism.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to fixation assemblies. More particularly,the present invention relates to screw fixation assemblies for use withbone fixation systems.

2. Background Art

Several techniques and systems have been developed for correcting andstabilizing the spine and for facilitating fusion at various levels ofthe spine. Stabilization of the spine for various conditions, includingdegenerative disk disease, scoliosis, spondylolisthesis, and spinalstenosis, often require attaching implants to the spine and thensecuring the implants to spinal rods. Such spinal fixation devices canimmobilize the vertebrae of the spine and can alter the alignment of thespine over a large number of vertebrae by connecting at least oneelongate rod to the sequence of selected vertebrae. These rods can spana large number of vertebrae, such as three or four. The spine anatomy,however, rarely allows for three or more implants to be directly inline. In order to allow for this irregularity, the rod must be contouredto the coronal plane.

Spinal fixation has become a common approach in fusion of vertebrae andtreating fractures and the above listed spinal disorders. A commondevice used for spinal fixation is a bone fixation plate assembly.Typical bone fixation plate assemblies have a relatively flat,rectangular plate with a plurality of apertures therethrough. Additionalassemblies include an implantation fixation system that locks a rod toseveral vertebrae. In these assemblies, as in with other spinal fixationsystems, they utilize various fasteners, such as bone screws, to securethe bone fixation plate assembly or the implantation fixation assemblyto the desired and targeted vertebrae of the patient. These screws varyin design and shape depending upon their desired location and usethereof.

In particular, polyaxial locking screws are used with these devices. Thekey to the polyaxial screws used with these systems is having the screwhead being securely fastened to the vertebrae and to the assemblythereof. Thus, the polyaxial screws must be used in conjunction with atype of screw head securing device that provides a strong lock to thepolyaxial screw. Any movement of the screw can be detrimental towardsthe healing process of the spine. Further, additional damage can occurif there is movement of the screw once it has been fixed to thevertebrae. Therefore, movement of the screw must be minimized oreliminated.

There are numerous polyaxial screws existing in the market today andknown in the prior art. Additionally, numerous devices exist thatprovide a securing means for locking the polyaxial screw. For example,U.S. Pat. No. 5,554,157, U.S. Pat. No. 5,549,608, and U.S. Pat. No.5,586,984 all to Errico et al. disclose polyaxial locking screws andcoupling element devices for use with a rod fixation apparatus. The '157patent discloses a coupling element including an interior axial passagehaving an interior surface that is inwardly curvate at the lower portionthereof such that it comprises a socket for polyaxially retaining aspherical head of a screw. The coupling element further includes a pairof vertically oriented opposing channels extending down from the top ofthe coupling element, which define therebetween a rod receiving seat.The channel further provides for walls of the upper portion to a pair ofupwardly extending members, each including an exterior threadingdisposed on the uppermost portion thereof for receiving a locking nut.During the implantation of the assembly, the locking nut seals againstthe top of the rod that in turn seats on top of the screw head. The nutcauses the rod to be locked between the nut and the screw and the screwto be locked in the socket.

The '608 patent discloses a modification wherein a locking ring isdisposed about the exterior of the lower portion of the coupling elementand provides an inward force on an outwardly tapered portion upondownward translation thereof. As a result, the interior chamber crushlocks a screw head therein to eliminate the polyaxial nature of thescrew element coupling.

The '984 patent discloses a polyaxial orthopedic device including acoupling element having a tapered lower portion having a slottedinterior chamber in which a curvate head of a screw is initiallypolyaxially disposed. The coupling element includes a recess forreceiving a rod of the implant apparatus. A locking ring is disposedabout the lower portion of the coupling element and provides an inwardforce on the outwardly tapered portion upon downward translationthereof. The vertical slots are caused to close and crush, therebylocking the screw head within the interior chamber thereof.

U.S. Pat. No. 6,280,442 to Barker et al. discloses a complex lockingmechanism having a screw head with a complex head geometry, a crownmember, and an outer rigid body. Locking occurs by compressing the crownmember against the complex head, which compresses the head against therigid seat. This compression crushes the machine ridges on the head andsecures the screw therein.

Another example of a common locking mechanism is a type of collet thathas a spherical seat with a flexible portion that is designed to deflectaround the screw. By compressing the flexible portion against a rigid,outer wall, the collet is compressed against the head to cause lockingtherein. Examples of these collets are found in numerous patents. Forexample, U.S. Pat. No. 6,053,917 to Sherman et al. discloses amultiaxial bone screw assembly that includes a bone screw having apartially spherical head. Additionally, the assembly includes a receivermember that has a central bore that defines a tapered recess to receivea contracting collet carrying the head of the bone screw. The colletdefines a partially spherical recess to receive the head of the bonescrew and includes deflectable fingers that substantially surround thescrew head. As a set screw is tightened into the receiver member, theset screw compresses the rod against the collet, which presses thecollet into the tapered recess of the receiver member, therebydeflecting the fingers of the collet against the bone screw head.

Another patent, U.S. Pat. No. 5,964,760 to Richelsoph, discloses aspinal implant fixation assembly that includes a bone fixation member. Arod receiving seat is operatively connected to the bone fixation elementfor seating a portion of a rod therein. A locking mechanism, in the formof a nut and locking ring, engages the rod receiving seat for forcing aninner wall of the rod receiving seat to contour around and engage therod seated therein and for locking and fixing the rod relative to theinner housing. The assembly further includes a screw head receivinginsert for obtaining a head of a screw therein. The insert is moveablewithin the assembly between a locked position entrapping the screw headand an unlocked position wherein the screw head enters or escapes.

Other polyaxial screw patents that utilize a similar collet aredisclosed in U.S. Pat. No. 6,010,503 to Richelsoph, U.S. Pat. No.5,910,142 to Tatar (disclosing the use of a spherical collet that iscompressed between the screw head and the rod), and U.S. Pat. No.5,891,145 to Morrison et al. (disclosing the use of a very complexdouble wedge locking mechanism).

More specifically, the '142 patent to Tatar discloses a polyaxialpedicle screw device for use with a rod implant apparatus, whichutilizes a rod mounted ferrule. The device further includes a screwhaving a curvate head and a rod receiving body. The body has a rodreceiving channel and an axial bore into which the head of the screw isinserted. The rod mounted ferrule seats into a small curvate recess inthe upper portion of the screw head such that the rod may enter the bodyat a variety of angles while maintaining secure seating against the headof the screw. The insertion of a top, set-screw compresses down on theferrule, locking the rod in position and onto the screw head. Further,the body is locked in position to completely secure the assembly.

The '145 patent to Morrison et al. discloses a spinal fixation assemblythat includes a bone engaging fastener and an elongated member such as aspinal rod. The fixation assembly is a multiaxial assembly that permitsfixation of the bone engaging fastener to the spinal rod at any of thecontinuous ranges of angles relative to the rod in three dimensionalspace. The fixation assembly includes a receiver member having a boretherethrough, the walls of which are tapered near the bottom, and achannel communicating with the bore and having an upper opening at thetop of the receiver member for insertion of a spinal rod. An outer wedgemember and an inner wedge member are also included. Both members havethe general shape of a washer and a bore therethrough. In each wedgemember, the respective bore is not parallel to the central axis of therespective wedge member. Additionally, the outside surfaces of the wedgemembers may be tapered and the respective bores may be tapered so as toself-lock when seated and tightened. The bone engaging fastener fitswithin the bore of the inner wedge member, which in turn fits within thebore of the outer wedge member, which in turn fits within the taperedsides of the receiver member. When the desired position of the boneengaging fastener in three dimensional space is attained, the componentsare seated to achieve a tight friction fit.

U.S. Pat. No. 6,063,089 to Errico et al. discloses a polyaxialorthopedic device for use with a rod implant apparatus that includes ascrew having a head, a tubular body having holes on top, side, andbottom thereof, and a rod coupling element. The head of the screw isdisposed in the body with the shaft of the screw extending out of thebottom hole, such that the body and the screw may initially rotaterelative to one another. The rod coupling element has a ball shaped endthat sits in the body with the remainder of the rod coupling elementextending out of the side hole of the body, such that the rod couplingelement and the body are initially polyaxially coupled relative to oneanother. The ball end of the rod coupling element is disposed on top ofthe head of the screw. A set screw is provided on top of the body, thetightening of which causes the ball, head, and body to be crushed lockedtogether, thereby preventing further relative motion.

In all of the existing prior art, particularly those described herein,the polyaxial screws utilize a complex locking mechanism and additionallocking parts to prevent the movement of the polyaxial screw. Typically,the more complex the locking mechanism, larger components are needed andmanufacturing costs are expensive. Locking and achieving strong holdvalues become more difficult with more parts. Further, the sizes of thevarious fixation plates and fixation assemblies are critical to thesetypes of surgeries. Bulky components can cause soft tissue irritation,as well as compromise the facet joints at the end of a fusion.Minimizing the size of the implants used is critical in spinal surgery.Soft tissue irritation resulting from extensions of implants is a commonoccurrence. Many times, it is caused by the implant being thick relativeto its environment. For example, implants can be too thick to besufficiently covered within the muscle tissue. Hence, a reduction in theoverall thickness of the implant is a critical advantage.

Accordingly, there is a need for a screw head securing mechanism ordevice that provides a strong, effective, and secure lock of the screwhead in its desired position. Additionally, there is a need for a screwhead securing mechanism or device that is minimal in size and has areduced amount of components to provide for a simpler, more effective,and less cumbersome device for fixing screws.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a screw and rodfixation assembly for fixing a screw and, optionally, a rod. The screwand rod fixation assembly includes a screw, fixing mechanism, asubstantially annular ring, rod seating mechanism, and lockingmechanism. The present invention also provides for a fixing mechanismfor fixing a screw, wherein the fixing mechanism further includes aninner surface wall having a gripping portion and a non-gripping portion.Further, the present invention provides for a substantially annular ringfor guiding and providing mechanical and frictional force to a screwhead. Additionally, the present invention provides for a rod seatingmechanism operatively engaged to the screw head and including at leastone flexible portion capable of being compressed against a portion of arod therein. Finally, the present invention provides for a lockingmechanism for engaging the rod and the rod seating mechanism. Thelocking mechanism includes a deflecting mechanism for deflecting the atleast one flexible portion of the rod seating mechanism against andaround the rod as the locking mechanism further engages the at least oneflexible portion of the rod seating mechanism.

DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention are readily appreciated as thesame becomes better understood by reference to the following detaileddescription when considered in connection with the accompanying drawingswherein:

FIG. 1 is a cross sectional view of one embodiment of a fixing mechanismof the present invention, wherein FIG. 1A shows a screw head (asindicated by the circle) that is freely moveable within a non-grippingportion of the fixing mechanism and FIG. 1B shows the screw headsecurely fixed within a gripping portion of the inner surface wall ofthe fixing mechanism of the present invention;

FIG. 2 is a detailed, cross-sectional view of an embodiment of a screwand rod fixation assembly of the present invention;

FIG. 3 is a detailed, cross-sectional view of an embodiment of thefixing mechanism, wherein the fixing mechanism is interiorly tapered ata distal end;

FIG. 4 illustrates another embodiment of the screw and rod fixationassembly where the screw head is securely fixed within the fixingmechanism, wherein a minimal amount of material is utilized for thefixing mechanism in order to minimize x-ray obstruction;

FIG. 5 illustrates another embodiment of the fixing mechanism thatutilizes composite polymer material in the outer shell with an innermetallic component to form a hybrid load sharing structure for securelyfixing the screw head within fixing mechanism;

FIG. 6 is a perspective view of a substantially annular ring used in anembodiment of the screw and rod fixation assembly wherein thesubstantially annular ring has a spherical indentation and a flatportion as well as a through hole and the insert can be any shape toprovide mechanical and/or frictional force to the screw head;

FIG. 7 is a perspective view of a rod seating mechanism including acylindrical and/or tapered outer surface having extending flexibleportions for engaging a spinal rod as a locking mechanism is tightenedupon the extending flexible portions, thus deflecting the extendingflexible portions around the rod situated therein;

FIG. 8 is a cross-sectional view of the rod seating mechanism shown inFIG. 7;

FIG. 9 is a perspective view of the locking mechanism (a set screw) foruse with the rod seating mechanism shown in FIG. 7, wherein the lockingmechanism has an inner chamber or spherical portion therein that engagesthe extending flexible portions of the locking mechanism to ensure thatthe extending flexible portions are deflected inward and contouredaround the rod seated therein;

FIG. 10 is a perspective view of an embodiment of the screw and rodfixation assembly including a screw, a fixing mechanism, the rod seatingmechanism, and the locking mechanism, wherein the screw head is texturedfor enhancing screw head locking within the fixing mechanism;

FIG. 11 is a perspective, line drawing of the screw and rod fixationassembly illustrated in FIG. 10;

FIG. 12A is an enlarged side-view of the rod seating mechanism of anembodiment of the present invention, wherein the rod seating mechanismseats the rod therein and the extending flexible portions of the rodseating mechanism are in an extended condition, while FIG. 12B is across-sectional view illustrating the rod seating mechanism with theextending flexible portions in a compressed condition contouring aroundand engaging against the rod thereon;

FIG. 13 is a perspective, side-view illustration of another embodimentof the screw and rod fixation assembly including a solid, substantiallyannular ring;

FIG. 14 is a perspective, side-view of the screw and rod fixationassembly including a substantially annular ring that is split to allowmaximum flexibility of the substantially annular ring within the fixingmechanism;

FIG. 15 is a perspective view of the fixing mechanism wherein an outersurface is a polymer, composite, or other material and the fixingmechanism includes a spherical screw seat that is situated within thegripping portion of the inner surface wall that also allows deflectiontherein;

FIG. 16 is a side-view of the fixing mechanism and rod seating mechanismof the present invention wherein the rod seating mechanism deflects overthe rod when pushed into the body of the fixing mechanism and tabs onthe side of the rod seating mechanism engage apertures or indentationsin the body of the fixing mechanism to ensure the rod seating mechanismremains locked;

FIG. 17 is a perspective view of another embodiment of the fixingmechanism wherein a threaded locking mechanism is pressed into thesubstantially tubular body of the fixing mechanism after the rod isintroduced into the fixing mechanism therein that engages a groove onthe inner surface wall and the locking mechanism is tightened to lockthe entire screw and rod fixation assembly; and

FIG. 18 is a cross-sectional view of an embodiment of the screw and rodfixation assembly wherein the head of a structure, as opposed to ascrew, is within the gripping portion of the fixing mechanism, andmovement of the head of the structure further into the gripping portionlocks the screw and rod fixation assembly.

DETAILED DESCRIPTION OF THE INVENTION

A screw and rod fixation assembly constructed in accordance with thepresent invention is generally indicated by 10 in the Figures.Generally, the screw and rod fixation assembly 10 fixes and/or locks ascrew, a rod, or both. Additionally, the screw and rod fixation assembly10 includes various components in different combinations. For instance,the screw and rod fixation assembly 10 can include any, all, orcombinations of a screw, fixing mechanism, substantially annular ring,rod seating mechanism, and locking mechanism.

The present invention provides for locking of a screw head without anymanipulation of the fixing mechanism thereof. The fixing mechanisminherently grips the screw head rigidly, preventing movement of thescrew head that can occur because of vertebrae movement, withoutapplication of force or manipulation of the fixing mechanism as requiredto ensure the fixed gripping. Rather, entry of the screw head into thelocking or gripping portion of the fixing mechanism causes locking tooccur around the screw head by means of compression force thereon.Basically, the fixing mechanism is a passageway including side-walls andan inherent locking mechanism for gripping and locking the screw head asthe screw head enters through the passageway and into the fixingmechanism. The passageway can be a substantially tubular body and becomposed of semi-flexible material.

The present invention has numerous advantages over the prior art. Theseadvantages include, but are not limited to, simplifying currentlyexisting polyaxial screw fixation assemblies. For instance, no expensivecollet or other similar screw head holding mechanism is used with thepresent invention. Additionally, the present invention is significantlysmaller in size, diameter, circumference, and overall dimensions thanother devices existing in the prior art. Because the present inventiondoes not utilize any additional components such as a collet, crown, orcap that exceeds the diameter of the screw head, the size of the presentinvention is greatly reduced. Thus, the present invention, and inparticular, the fixing mechanism, only needs to be large enough to allowthe screw to enter or be seated therein. Another advantage of thepresent invention is that the screw and/or screw head is secured moreuniformly and in a more controlled manner.

Further, the tolerance issues of collet designs, crowns, caps, andcomplex screw retaining mechanism geometries are eliminated. Moreover,the screw placed within the present invention can be securely lockedwithout any internal mechanism to create a fixed screw. An instrumentcan easily push down the screw to the proper seating position to createa locked screw at any desired angle, even after the instrument isremoved.

The term “elastic range” as used herein is the limit of stress withinwhich deformation of a body of material completely disappears after theremoval of stress, tension, and/or force upon the body of material.Elasticity is the ability of a material to return to its originaldimensions after removal of stress, tension, or force placed upon thematerial. Once the stress, tension, or forces exceed the elastic rangehowever, the material cannot return to its original dimensions and isforever deformed.

The term “deformation range” as used herein means the excessive amountof strain, force, or load outside the elastic range wherein deformationis maintained and the body of material no longer can return to itsoriginal dimensions. Basically, deformation occurs wherein a change inform is produced by external forces or loads that act on the body ofmaterial.

The term “screw” 12 as used herein means a simple machine of theinclined plane type including a spirally grooved solid cylinder and acorrespondingly grooved hollow cylinder into which it fits. The screw 12can be any type of screw such as set screw and can be any size, shape ordesign as is known to those of skill in the art. In certain cases, itcan be desirable to have the screw 12 have a screw head 14 of anothershape and/or size.

The term “rod” 16 as used herein means a substantially cylindrical bodyhaving a desired length. With regard to the present invention, the rod16 is typically a rod 16 used within a human body to aid in thealignment of bones, particularly the vertebrae.

One important feature of the screw fixation assembly 10, and moreparticularly the fixing mechanism, is the use of a semi-flexiblematerial as opposed to a rigid material. As is known to those of skillin the material strength art, materials such as metals and compositesthereof have an elastic range and a deformation range. For example, if asample of a material is placed in tension and not allowed to exceed theelastic range, the sample will stretch.

Elastic stretching occurs in such a manner that upon release of thetension, the material will return to its original dimensions. Byadapting this property to the present invention, the wall of the fixingmechanism acts as a controllable spring that can effectively be used toexert a uniform compressive load on a head portion of a screw, shaft, orother component. The elastic stretching occurs by introducing a headportion that is larger than the internal dimensions of the fixingmechanism. This forces the fixing mechanism to expand to accept the headportion. In a similar fashion as a spring, the fixing mechanism wouldreturn to its original dimensions upon removal of the head portion.However, while the head portion is within the fixing mechanism, thefixing mechanism exerts a compressive force on the head portion. Thus,the fixing mechanism operates by inherently gripping the screw headrigidly, preventing movement of the screw head without the applicationof force or manipulation of the fixing mechanism by another means.

This property of the expansion of the wall of the fixing mechanismwithin its elastic range and the resultant force generated inward aroundthe head portion is referred to as a “spring reaction.” Outside of thiselastic range, the material permanently deforms and will not return toits original dimensions. Therefore, it is essential to stay within theelastic range of the material in order to have a controllable reactionin which to use the compressive forces created therein.

The compressive forces created by the expansion of the wall of thefixing mechanism can be readily controlled by altering the material,material thickness, coatings, or otherwise modifying the material of thefixing mechanism. The head portion can also be altered to be of anymaterial and hardness. A softer material would allow for morecompensation for tolerances on the dimensions of the components, while aharder material could provide a more controlled expansion of the fixingmechanism.

Entry of the screw head into the gripping or locking portion of thefixing mechanism causes gripping or locking to occur around the screwhead by means of compression force thereon. Generally, the fixingmechanism is a passageway including side-walls and an inherent lockingmechanism for gripping and locking the screw head as the screw headenters through the passageway and into the locking mechanism. Thepassageway is a substantially tubular body, while the inherent lockingmechanism is a semi-flexible portion of the side-wall. Another manner inwhich to describe the fixing mechanism is that it is a single-walledvessel made of semi-flexible material.

The present invention can be constructed from any suitable materialknown to those of skill in the art. Preferably, the present invention isconstructed from material that is compatible with use in theenvironments into which they are placed. The present invention can beconstructed of various metallic materials that include, but are notlimited to, titanium, stainless steel, and any other suitable metallicalloys known to those of skill in the art. Additional materials can alsobe utilized either alone or in combination with the above-describedmetallic materials. For instance, various plastics, fibers, and otherpolymers can be used in conjunction with the various metallic materials.These combinations can be used to construct various components orportions of the present invention. Moreover, other types of coatings canbe placed on various portions of the present invention in order toimprove durability, strength, and utilization thereof. Typically though,any of the material used to construct the present invention should bevery strong, non-reactive, and non-antigenic to biological systems ifthey are used in that environment. If the present invention is usedoutside of biological systems however, the aforementionedcharacteristics are not necessarily required.

There are numerous embodiments of the present invention. As such,similar structures amongst the several embodiments are shown by primenumbers in the various Figures. Although there are numerous embodimentsdisclosed and described herein, each of the embodiments includes avariation of the fixing mechanism of the present invention.

In the first embodiment of the present invention, the screw and rodfixation assembly 10 includes a screw 12 having a screw head 14. Thescrew and rod fixation assembly 10, and more specifically the fixingmechanism 18, grips and secures the screw head 14. Optionally, the screwand rod fixation assembly fixes a rod 16. The screw and rod fixationassembly 10 includes the fixing mechanism 18 for fixing the screw head14 from movement relative to the screw and rod fixation assembly 10. Thefixing mechanism 18 includes an inner surface wall 20 having a grippingportion 22 and a non-gripping portion 24. The fixing mechanism 18operatively engages the screw head 14 and ultimately the screw 12. Thefixing mechanism is basically a passageway or single-walled vessel madeof semi-flexible material. As described above, the fixing mechanism 18,and more particularly the gripping portion 22, securely grips the screwhead 14 through compression forces generated by the spring reaction ofthe semi-flexible material. To reiterate, the spring reaction occursfrom the screw head 14 stretching the gripping portion 22 beyond itsoriginal dimensions, but within the material's elastic range. Thus, thegripping portion 22 has a spring reaction to return back to its originaldimensions that results in creating compression forces onto the screwhead 14. As a result, the screw head 14 is securely gripped within thegripping portion 22.

The screw and rod fixation assembly 10 includes a substantially annularring 26 for guiding and providing mechanical and frictional force to thescrew head 14. Moreover, the screw fixation assembly 10 includes a rodseating mechanism 28 operatively engaged to the screw head 14 and has atleast one flexible portion 46 capable of being compressed against therod 16 seated within the rod seating mechanism 28. The rod seatingmechanism 28 also includes a tapered outer surface end 34.Alternatively, the rod seating mechanism 28 can be cylindrically shaped.

The screw and rod fixation assembly 10 includes a locking mechanism,generally shown at 36 in the Figures. The locking mechanism 36 engagesthe rod 16 and the rod seating mechanism 28. The locking mechanism 36includes a deflecting mechanism 38 for deflecting at least one flexibleportion 46 of the rod seating mechanism 28 against and around the rod 16as the locking mechanism 36 further engages the at least one flexibleportion 46 of the rod seating mechanism 28. The deflecting mechanism 38specifically engages and deflects the outer tapered surface end 34therein.

The fixing mechanism 18 includes a substantially tubular body 40. Aspreviously stated, the fixing mechanism 18 includes the gripping portion22 and the non-gripping portion 24. It is this fixing mechanism 18 thatis included in all of the various embodiments described herein. Aspreviously described, the fixing mechanism 18, specifically the grippingportion 22, utilizes semi-flexible material that creates compressionforces on the screw head 14 that is engaged with the gripping portion 22when the gripping portion 22 is stretched beyond its original dimensionsand returns to its original dimensions. The gripping portion 22 is aportion of the substantially tubular body 40 that grips and secures thescrew head 14 once the screw 12 is inserted into the gripping portion22. The gripping portion 22 grips and secures the screw head 14 by thespring reaction of the material composing the fixing mechanism 18. Thus,the screw head 14 of the screw 12 extends the inner surface wall 20within its elastic range so that it is not permanently deformed. As aresult of the elasticity of the material of the inner surface wall 20,the spring reaction of the material creates enough force upon the screwhead 14 to securely fix the screw head 14 and ultimately the screw 12 inplace.

As for the non-gripping portion 24, the screw 12 and the screw head 14do not extend the inner surface wall 20 therein. Instead, the screw 12and screw head 14 are freely moveable within the area surrounded by thenon-gripping portion 24 of the inner surface wall 20 of the fixingmechanism 18. The gripping portion 22 and non-gripping portion 24 of theinner surface wall 20 can be made of the same material or differentmaterials depending upon the design. For instance, the gripping portion22 can have an added coating or material in order to provide for addedstrength and/or flexibility. Preferably, the entire fixing mechanism 18is made from semi-flexible material, especially at the gripping portion22 end. Of course, the non-gripping portion 24 can also be made of asemi-flexible material. The semi-flexible material includes, but is notlimited to, metal, plastics, alloys, polymers, fibers, combinationsthereof, and any other similar materials that have a desired elasticityand elastic range. Thus, the locking of the screw head 14 requires noother manipulation of the fixing mechanism than the screw head 14entering the gripping portion 22 of the substantially tubular body 40.

The substantially tubular body 40 of the fixing mechanism 18 can havevarious designs. Although a substantially tubular body 40 is utilizedwith the present invention, any other similar body shapes known to thoseof skill in the art can be utilized with the present invention. Thesubstantially tubular body 40 also varies in overall dimensionsdepending upon the desired use thereof. Further, the substantiallytubular body 40 can be segmented so portions of the substantiallytubular body 40 can be snapped off. Further, the substantially tubularbody 40 can be threaded or smooth. The threading can be on the exteriorsurface or on the interior surface thereof. The threading is useful whenthe fixing mechanism 18 is used in conjunction with a similarly threadedlocking mechanism 36 as is described in more detail below.

An example of the fixing mechanism 18 is illustrated in FIGS. 1A and 1B.In this embodiment, the fixing mechanism 18 utilizes a relativelythin-walled, substantially tubular body 40. The substantially tubularbody 40 has an outer diameter and an inner diameter such that a portionof the inner diameter is smaller in diameter of the screw head 14 (i.e.,gripping portion 22 of the inner surface wall 20). When the screw head14 is pushed downward into the substantially tubular body 40, the screwhead 14 forces the smaller inner diameter to expand, but to remainwithin its elastic range. With the inner surface wall 20 remainingelastic, the spring reaction of the inner surface wall 20 at thegripping portion 22 creates considerable compression force as the innersurface wall 20 naturally returns from a deformed state to its originalstate. This spring reaction or force is equivalent to a mechanicalvariant of a pressure vessel creating a hoop stress in the inner surfacewall 20. As long as this stress remains within the elastic range of thematerial of the inner surface wall 20, the load or force upon the screwhead 14 remains constant. In addition, the force exerted on the screwhead 14 is directly related to the ability of the gripping portion 22 ofthe inner surface wall 20 to resist deflection or expansion in thiscase. Thus, the thicker the inner surface wall 20 at the grippingportion 22, the more spring reaction and compression forces aregenerated and applied to the screw head 14. Of course, the further thescrew head 14 has to be pushed into the smaller diameter area (grippingportion 22) and the thicker the inner surface wall 20 therein, the moreforce is required to drive the screw head 14 further into the smalldiameter area (gripping portion 22).

To one skilled in the art, there are various ways to narrow the innerdiameter of the fixing mechanism 18. One method is to reduce the innerdiameter by simple boring techniques (FIGS. 1A and 1B). Additionally,various molds can be created to form the same design.

In another embodiment of a fixing mechanism 18′, the inner surface wallis interiorly tapered therein and the interior surface wall 20′ is alsothreaded 25. (FIG. 2). As is shown in FIG. 2, the fixing mechanism 18′is a substantially tubular body 40 or single-walled vessel. The fixingmechanism 18′, more particularly the gripping portion 22′, is made ofthe semi-flexible material described above. The gripping portion 22′,due to the semi-flexible material, creates enough compression force ontothe screw head 14 to fix it therein when the gripping portion 22′ isstretched beyond its original dimensions by the screw head 14. Thegripping portion 22′ has a spring reaction when it returns to itsoriginal dimensions after being temporarily deformed by the screw head14 after the screw head enters the interior portion 23 of the grippingportion 22′ therein.

The interior of the gripping portion 22′ of the fixing mechanism 18′ isspherically shaped 23 therein. Thus, as the locking mechanism 36′ isinserted within the fixing mechanism 18′, the screw head 14 engages thespherically shaped 23 gripping portion 22′ therein. The more the screwhead 14 is pushed therein, the more interference is generated, which iscompensated for by the expansion of the tubular structure to create thecompressive force onto the screw head 14 (e.g., spring reaction). Inthis embodiment, the fixing mechanism 18′ includes a pair of spaced,substantially parallel arms 42 extending from the substantially tubularbody 40′. These arms 42, as is shown in FIG. 2, can be optionallysnapped-off from the substantially tubular body 40′ after assembly ofthe screw fixation assembly 10′ is completed.

In FIG. 3, another embodiment of the fixing mechanism 18″ is illustratedtherein. This embodiment is similar to the embodiment illustrated inFIG. 2; however, this embodiment of the fixing mechanism 18″ has athreaded exterior surface 27 as opposed to a threaded interior surface25. Again, the fixing mechanism 18″, and more particularly the grippingportion 22″, is made of the previously described semi-flexible materialwherein the gripping portion 22″ creates compression forces on the screwhead 14 when the gripping portion 22″ returns to its original dimensionsafter being stretched by the screw head 14 inserted therein. Theembodiment of the fixing mechanism 18″ illustrated in FIG. 3 uses adifferent locking mechanism 36″ that is similar to a nut or otherdevice. The embodiment of the fixing mechanism 18″ in FIG. 3 alsoincludes a pair of spaced, substantially parallel arms 42′ extendingfrom the substantially tubular body 40″ therefrom.

In yet another embodiment of the fixing mechanism 18′″ (FIG. 4), metalor metal and composite material (e.g., polymers) are utilized. Thecomposite material can be blended with the metal or be used as an outercoating shell. The composite material is still semi-flexible so thatcompression forces are generated when the gripping portion 22′″ isstretched, but returns to its original dimensions. Further, the grippingportion 22′″ of the fixing mechanism 18′″ illustrated therein uses aminimal amount of material to minimize x-ray obstruction while stillproviding sufficient spring reaction in order to secure the screw headtherein. The screw head 14 used with this particular fixing mechanism18′″ is spherically shaped.

FIG. 5 illustrates another embodiment of the fixing mechanism 18″″ suchthat a composite material is used. The fixing mechanism 18″″ includes acomposite/polymer outer shell having an inner metallic component to forma hybrid load sharing structure. Again, the fixing mechanism 18″″ andmore particularly the gripping portion 22″″, utilizes semi-flexiblematerial so that sufficient compression forces are generated on thescrew head 14 when the gripping portion 22″″ returns to its originaldimensions after being stretched by the screw head 14 inserted therein.The fixing mechanism 18″″, as is shown in FIG. 5, is basically apassageway or single-walled vessel that can accommodate the screw head14 therein.

In FIG. 15, another embodiment of the fixing mechanism 18′″″ is shown.The fixing mechanism 18′″″ is composed of a polymer that issemi-flexible. The fixing mechanism 18′″″ includes a substantiallytubular body 40′″″ with a metallic ring at the screw head 14 location.As previously described, the semi-flexible fixing mechanism 18′″″, andmore specifically the gripping portion 22′″″, generates compressionforces onto the screw head 14 when the gripping portion 22′″″ returns toits original dimensions after being stretched by the screw head 14. Thefixing mechanism 18′″″ further includes a spherical screw seat 23. Thespherical screw seat 23 generally is machined into the substantiallytubular body 40′″″ therein. The spherical screw seat 23 allowsdeflection and grip. This embodiment is very useful in cases where thesubstantially tubular body 40′″″ is not strong enough to provide amplespring reaction or force on the screw head 14 or enough frictionalholding force. This is an example of a load sharing composite structure.

Optionally, the screw and rod fixation assembly 10 further can include asubstantially annular ring 26 having an edge portion 30 extending abouta centered axis. The annular ring 26 operates in conjunction with thefixing mechanism 18 that includes a substantially tubular body 40forming a passageway or vessel described herein. While the fixingmechanism 18 does create sufficient compression forces due to the springreaction of the material composing the fixing mechanism 18 and morespecifically the gripping portion 22 (the spring reaction generatescompression forces when the gripping portion is returning to itsoriginal dimensions after being stretched), there are times whenadditional frictional and/or mechanical forces are required.

The edge portion 30 of the annular ring 26 has a frustoconical surfacetapering outwardly toward the edge portion 30 for engaging a screw head14, while allowing a portion of the screw 12 to pass therethrough. Thesubstantially annular ring 26 guides the screw head 14 and providesadditional mechanical and frictional forces onto the screw head 14. Thescrew and rod fixation assembly 10 requires both mechanical andfrictional locking. Holding the rod 16 requires mostly frictionallocking, while securing the screw head 14 requires both mechanical andfrictional locking. The various material utilized to compose the fixingmechanism 18 cannot necessarily provide sufficient mechanical andfrictional locking force. Thus, by using a smaller, substantiallyannular ring 26 within the fixing mechanism 18, sufficient mechanicaland frictional locking force can be provided therein. The locking can beshared by the metallic portion of the substantially annular ring 26,while the polymer/fiber reinforced portion creates a stronger interface.The use of the substantially annular ring 26 is optional. Thus, thescrew and rod fixation assembly 10 can include it. Alternatively, anembodiment of the screw and rod fixation assembly 10 can exclude it sothat the screw and rod fixation assembly 10 includes only a screw 14, afixing mechanism 18, a rod seating mechanism 28, and a locking mechanism36.

The present invention further provides for a rod seating mechanism 28operatively engaged with the fixing mechanism 18. The rod seatingmechanism 28 further includes at least one flexible portion 46 capableof being compressed against the rod 16 seated within the rod seatingmechanism 28. The flexible portion 46 has a tapered outer surface end34. The rod seating mechanism 28 has a body portion 44. In theembodiments shown in FIGS. 7, 10-14, and 16, there are two flexibleportions or arms 46 that are substantially parallel to each other. Inthose situations, the two flexible portions 46 and body portion 44 forma substantially U-shaped inner wall 32 defining the first seat 30therein. Further, the flexible portions 46 include a tapered outersurface end 34.

The at least one flexible portion 46 of the rod seating mechanism 28securely engages the rod 16 therein by contouring around the outercircumference or surface of the rod 16. The rod seating mechanism 28 isaltered to allow the deflectable flexible portions 46 to engage the rod16 as the locking mechanism 36 is tightened thereon. (FIGS. 7 and 8).Thus, as the locking mechanism 36, as illustrated in FIG. 9, is advancedwithin the fixing mechanism 18, the flexible portions are pushed overthe top of the rod 16 thereby increasing contact area of the flexibleportions 46 over the rod 16 and further increasing frictional forcethereon. (FIGS. 12A and 12B). FIG. 12A illustrates the rod seatingmechanism 28 wherein the rod 16 is seated therein and the flexibleportions 46 are extended above the rod 16. FIG. 12B illustrates the rodseating mechanism 28 having the flexible portions 46 being compressedagainst and around the rod 16 as the result of the locking mechanism 36being tightened around the flexible portions 46. The flexible portions46 are deflected inward by the locking mechanism 36 in order to compressthe flexible portions 46 around and against the rod 16. In FIGS. 12A and12B, the locking mechanism 36 is not shown in order to clarify andfurther illustrate the bending of the flexible portions 46.

As previously mentioned, it is possible to add a substantially annularring 26 or internal sleeve 26 to enhance grip or act as a spacer betweenthe fixing mechanism 18 and screw head 14 therein. When grip isenhanced, a material having a softer or different composition than theouter surface wall 21 of the substantially tubular body 40 can be placedon the inner surface wall 20 for the screw head 14 to compress againstrather than directly against the outer surface wall 21. For example, asoft, commercial pure substantially annular ring 26 or sleeve 26 isinside a stronger body of Ti-6Al-4V. In this case, the outer surface 21acts as a tubular spring, but the softer inside, which would bepotentially too weak to allow the spring reaction, would allow the screwhead 14 to bite into the softer material for enhanced screw locking.(FIGS. 13 and 14). In addition, for manufacturing, it can beadvantageous to overbore the inside of the substantially tubular body 40and add a substantially annular ring 26 or sleeve 26. Such a sleeve 26can be a thin, solid wall or a split wall having a gap 29 as shown inFIGS. 13 and 14 respectively. The split or gap 29 allows the sleeve 26to flex without interfering with the outer surface 21 of thesubstantially tubular body 40. The sleeve 26 can be pressed fit or havesome clearance for easier assembly. A semi-flexible polymer or compositematerial can be used for the substantially tubular body 40 and adifferent material such as titanium can be used for the sleeve 26 toprovide for either additional reinforcement, better screw 12 locking,and/or improved rod 16 engagement.

With regard to the locking mechanism 36 of the present invention, thelocking mechanism 36 engages a rod 16 and rod receiving mechanism 28.The locking mechanism works in conjunction with the fixing mechanism 18wherein the fixing mechanism 18 is typically a substantially tubularbody 40 that can accommodate the locking mechanism 36 therein. Again,the fixing mechanism 18 is basically a passageway or single-walledvessel made of semi-flexible material. The fixing mechanism 18, throughthe gripping portion 22, places compression forces onto the screw head14 as a result of the spring reaction created when the gripping portion22 returns to its original dimensions after being stretched within itselastic range by the screw head 14 inserted therein. The lockingmechanism 36 secures and tightens the entire screw and rod fixationassembly 10. The locking mechanism 36 includes a deflecting mechanism 38for deflecting at least one flexible portion 46 of the rod seatingmechanism 28 against and around the rod 16 as said locking mechanism 36further engages the at least one flexible portion 46 of the rod seatingmechanism 28. The deflecting mechanism 38 can be an inner surface orportion of the locking mechanism 36 itself. The deflecting mechanism 38further engages the tapered outer surface 34 therein.

The locking mechanism 36 can be a nut, screw, set screw, or othersimilar locking mechanism 36 as is known to those skilled in the art.The locking mechanism 36 usually is threaded on at least one surfacethereon. FIGS. 9, 14, 10, 11, 13, and 14 illustrate embodiments of thelocking mechanism 36. As stated, the locking mechanism 36 is preferablya set screw including a threaded outer surface 48 for operativelyengaging an inner threaded surface 25 of the fixing mechanism 18.Alternatively, the locking mechanism 36 can be a locking nut including athreaded inner surface 50 for operatively engaging an outer threadedsurface 27 of the fixing mechanism 18 as disclosed herein.

The locking mechanism 36 can further include a spherical, inner chamber39 for accommodating at least one flexible portion 46 of the rod seatingmechanism 28 when the at least one flexible portion 46 is contouredaround the rod 16 situated therein. The spherical, inner chamber 39 canalso be useful in accommodating any type of instrument.

In other variations of the present invention, a locking mechanism 36 isnot included; instead, a different type of rod seating mechanism 28″″″can be used. In this particular embodiment, the rod seating mechanism28″″″ is inserted directly into the fixing mechanism 18″″″. The rodseating mechanism 28″″″ operatively engages the fixing mechanism 18″″″.Again, the fixing mechanism 18″″″ is made of semi-flexible material thatgenerates compression forces that are applied to the screw head 14 whenthe fixing mechanism 18 (more specifically the gripping portion 22″″″)returns to its original dimensions after being stretched within itselastic range by the screw head 14 inserted therein.

The rod seating mechanism 28″″″ includes the same structural componentsof the rod seating mechanism 28 described above. The rod seatingmechanism 28″″″ includes at least one flexible portion 46″″″ having atapered outer surface 34″″″ with at least one tab mechanism 31 radiallyextending from the tapered outer surface 34″″″ for operatively engagingand inserting into at least one indentation 33 or aperture 35 situatedon the inner surface wall 24″″″ of the substantially tubular body 40″″″of the fixing mechanism 18″″″. The size, shape, and dimensions of thetab mechanism 31 varies according to a desired size of the overallstructure of the rod receiving mechanism 28″″″. Likewise, theindentation 33 or aperture 35 extending through the inner surface wall24″″″ varies according to design. (FIG. 16)

As the rod seating mechanism 28 ″″″ is inserted into the fixingmechanism 18″″″ and over the screw head 14, the flexible portions 46″″″of the rod seating mechanism 28″″″ contour around and compress theinserted rod 16 situated therein due to the deflection of the tabmechanisms 31 caused by interference created by the inner surface wall24″″″ of the fixing mechanism 18″″″. Once the tab mechanisms 31 engagewith either the indentation 33 or the aperture 35, the flexible portions46′″″ of the rod seating mechanism 28′″″ remain locked in a compressedcondition contoured around the rod 16. In order to release the rodseating mechanism 26′″″ from the compressed condition, an instrumentmerely needs to be inserted between the rod seating mechanism 26″″″ andthe fixing mechanism 18″″″.

In yet another variation of the present invention, the rod seatingmechanism 28 is eliminated and replaced by a different embodiment of thelocking mechanism 36′″″″ wherein the locking mechanism 36′″″″ is a plugsecuring mechanism 52 (FIG. 17) that is inserted into the screw and rodfixation assembly 10 and onto the rod 16 after the rod 16 is introducedinto the fixing mechanism 18′″″″. As with the other embodiments, thefixing mechanism 18′″″″ is made of semi-flexible material. Again, due tothe semi-flexible material, the fixing mechanism 18′″″″, andparticularly the gripping portion 22′″″″, generates compression forcesthat are applied to the screw head 14 once the gripping portion 22′″″″returns to its original dimensions after being stretched within itselastic range.

The plug securing mechanism 52 can be any screw type device such as aset screw 36″″″. The plug securing mechanism 52 contacts at least aportion or part of the rod 16 in order to securely fix the rod 16situated therein and the entire screw and rod fixation assembly 10′″″″.The plug securing mechanism 52 is substantially cylindrical in shape andis capable of insertion into the substantially tubular body 40 of thefixing mechanism 18′″″″. The plug securing mechanism 52 also includes atleast one outwardly tapered extending tab mechanism 55 for locking theplug securing mechanism 53 onto the rod 16. The extending tab mechanism55 operatively engages at least one indentation 33′ or aperture 35′situated within the inner surface wall 20′″″″ of the substantiallytubular body 40′″″″ of the fixing mechanism 18′″″″. The size, shape, anddimensions of the extending tab mechanism 55 vary according to a desiredsize of the overall structure of the fixing mechanism 18′″″″. Likewise,the indentation 33′ on the inner surface wall 20′″″″ or the aperture 35′extending therethrough the inner surface wall 20′″″″ varies accordingly.(FIG. 17)

As the plug securing mechanism 52 is inserted into the fixing mechanism18′″″″ and over the rod 16, the plug securing mechanism 52 compressesagainst the inserted rod 16 situated within the first seat 30′″″″. Then,when the extending tab mechanisms 55 engage with either the indentation33′ or the aperture 35′, the plug securing mechanism 52 remains lockedin a compressed condition onto the rod 16.

Optionally, the plug securing mechanism 52 further includes a threadedinner surface 53 for operatively engaging any type of additional screw12 for further tightening of the rod 16. Thus, a screw 12 can beinserted and screwed into the plug mechanism 52. Alternatively, theouter surface 57 of the plug securing mechanism 52 can be threaded forengaging the threaded inner surface wall 20′″″″ of the substantiallytubular body 40′″″″ of the fixing mechanism 18″″″.

The present invention is not limited for use with just a bone screw 12.For instance, the fixing mechanism 18″″″″ can be extended to be usedwith other types of screws, mechanical parts, and/or structures. Thefixing mechanism 18″″″″ still utilizes, as with the other embodiments,semi-flexible material to create compression forces resulting from thespring reaction created when the fixing mechanism 18″″″″ returns to itsoriginal dimensions after being stretched within its elastic range.

The other parts that can be used with the present invention includeother implants, such as a swivel joint 58 for a cross-link connectorthat can be fixed with the present invention. (FIG. 18). In thissituation, a spherical head 54 is forced into a reduced diameter section56 that exerts a force on the spherical head 54 to lock it securely. InFIG. 18, the spherical head within the tube portion is shown andmovement of the spherical head into the gripping portion 22 locks theentire assembly. The assembly 10″″″″ as shown in FIG. 18 can be furtherlocked by the locking mechanism 36 as described herein. Additionally, aninstrument can be utilized to simply lock the assembly 10. Simplypulling on the two components with ample force in opposite directionswill lock the two components securely as the wall expands and exertscompressive forces on the head portion as illustrated therein in FIG.19.

The components for the screw fixation assembly and device disclosed anddescribed herein can be manufactured by various methods known to thoseof skill in the art. For instance, the assembly and device can be madeby first blanking the outer shape from a round bar stock. Then, byholding on the threaded end or an extension of the threaded end, a holeis made in the opposite end. This hole is undersized relative to thetaper to allow the taper to be cut with a single tool. While the partturns in a lathe, a boring bar having a small cutting tip is introducedinto the hole and the taper and recess cut. The threads are then cut,any extension cut off, and the slot either milled or cut to be morecompatible.

The various other components can be made by cutting the outsidecylindrical shape with an extension to hold on in a lathe. A hole isdrilled into one end and a boring bar with a small cutting tip is usedto enter the hole and cut the spherical seat. The outer slots and hingedetails are cut by either slitting away with a saw or a wire.

Throughout this application various publications are referenced byauthor, and year. United States patents however, are referenced bynumber and inventor. Full citations for the publications are listedbelow. The disclosures of these publications and patents in theirentireties are hereby incorporated by reference into this application tomore fully describe the state of the art to which this inventionpertains.

The invention has been described in an illustrative manner, and it is tobe understood that the terminology that has been used is intended to bein the nature of words of description rather than of limitation.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is, therefore, to beunderstood that within the scope of the appended claims, the inventioncan be practiced otherwise than as specifically described.

1-31. (canceled)
 32. A screw and rod fixation assembly for fixing ascrew and a rod comprising: a screw including a screw head; a fixingmechanism including an inner passageway for receiving said screwtherethrough for automatically and compressively fixing said screw frommovement relative to said fixing mechanism, said fixing mechanismincluding an inner surface wall having a gripping portion forcompressively gripping said screw head and a non-gripping portionallowing passage of the screw head to said gripping portion; a rod seatdisposed inside the inner passageway of the fixing mechanism and havinga U-shaped inner wall for seating a rod therein, said rod seat includingat least one flexible portion capable of being compressed against a rodseated within said rod seat, wherein said at least one flexible portionhas a tapered outer surface end; and locking means disposed inside theinner passageway of the fixing mechanism and engaging the rod and rodseat, said locking means including a deflecting surface for deflectingsaid at least one flexible portion of said rod seat against and aroundthe rod as said locking means further engages said at least one flexibleportion of said rod seat.
 33. The screw and rod fixation assemblyaccording to claim 32, wherein said fixing mechanism includes asubstantially tubular body.
 34. The screw and rod fixation assemblyaccording to claim 33, wherein said substantially tubular body furtherincludes spaced, substantially parallel arms extending from saidsubstantially tubular body.
 35. The screw and rod fixation assemblyaccording to claim 32, wherein said gripping portion of said innersurface wall of said fixing mechanism is expanded within an elasticrange of said inner surface wall by said screw head.
 36. The screw androd fixation assembly according to claim 32, wherein a portion of saidinner surface wall of said fixing mechanism is threaded.
 37. The screwand rod fixation assembly according to claim 32, wherein said rod seatincludes a body portion from which at least one said flexible portionextends therefrom.
 38. The screw and rod fixation assembly according toclaim 32, wherein said locking means is defined as a set screw includinga threaded outer surface for engaging said threaded inner surface ofsaid fixing mechanism.
 39. The screw and rod fixation assembly accordingto claim 38, wherein said set screw further includes a spherical, innerchamber for accommodating said at least one flexible portion when saidat least one flexible portion is contoured around the rod situatedtherein.
 40. A rod fixation assembly for fixing a screw and a rodcomprising: a fixing mechanism having a substantially tubular body withan inner passageway and an inner wall surrounding the inner passageway;a rod seat inserted into the inner passageway, the rod seat having apair of flexible portions extending generally parallel to one anotherforming a U-shaped inner surface adapted for seating a rod between theflexible portions, the flexible portions being deflectable radiallyinwardly toward one another to compress against and around the exteriorof a rod seated in the rod seat.
 41. The rod fixation assembly of claim40 comprising a locking nut in cooperable engagement with the fixingmechanism, the locking nut being axially displaceable to engage theflexible portions of the rod seat and compress the flexible portionsradially inwardly to compress the flexible portions against and aroundthe exterior of a rod seated in the rod seat.
 42. A fixing mechanism fora rod fixation assembly comprising a tubular body and a pair of armsextending in a substantially parallel arrangement, the arms beingseparated from one another by a passageway extending along a portion ofthe tubular body, said passageway adapted to receive a rod securingfastener inside the tubular body, each arm comprising a thinned sectionadapted for breaking off a portion of the arm from the fixing mechanism,the thinned sections extending within a plane that is substantiallyperpendicular to the longitudinal axis of the tubular body.
 43. Thefixing mechanism of claim 42, wherein the tubular body includes an innersurface with inwardly-facing threads.
 44. The fixing mechanism of claim42 comprising a rod seat inserted in the passageway of the tubular body,the rod seat including a pair of parallel extensions forming a U-shapedsurface for seating a rod between the parallel extensions.
 45. Thefixing mechanism of claim 44, wherein the arms of the tubular body areseparated by a pair of channels, each channel having a U-shaped end. 46.A fixing mechanism for a rod fixation assembly comprising asubstantially tubular body having a pair of arms extending in asubstantially parallel arrangement, the arms being separated from oneanother by a passageway extending along a portion of the tubular body,each arm being segmented into a first portion and a second portion, thesecond portion being adapted to be snapped-off from the first portion,the first portion having an axial length substantially equal to theaxial length of the second portion.
 47. The fixing mechanism of claim46, wherein the first and second portions of each arm are separated fromone another by a frangible section.
 48. The fixing mechanism of claim46, wherein the tubular body comprises an inner surface withinwardly-facing threads, the inwardly-facing threads being adapted toengage and axially advance a rod securing fastener that is inserted inthe passageway.